A Certain Unknown Mob
by ahsoei
Summary: Kageyama Shigeo always believed that his psychic power is not necessary in his every day life. What will happen when the Mob is suddenly offered a scholarship into a school within a certain city where psychic powers are everything? Enter the latest level-5 gemstone. Possible pairings. Rating and character details might change in the future. OPEN FOR ADOPTION
1. Prologue

**A Certain Scientific Mob**

* * *

 **What's up, everyone? Here's the prototype of my recent project, To Aru Majutsu no Index and Mob-Psycho 100 crossover. Without further ado, enjoy the prologue!**

* * *

 **Prologue: Enter the Mob**

In the middle of District 7 of Academy City, there was a typical Japanese middle school on one of the streets. It was morning; students in uniforms, boys and girls alike were walking and heading towards their respective classrooms. Among those students, one individual was rather standing out.

She was a small girl who sported short black hair and had golden-brown eyes. She had thick eyebrows but her most striking feature was her headband made out of artificial flowers. The girl was walking in steady pace while humming at her cell-phone, reading at whatever was displayed on the screen. She then smiled at the phone before putting it away inside her pocket, when someone suddenly appeared behind her –

"U-i-ha-ru~!"

And her skirt was flipped, revealing her lower undergarment for the world to see. It took few seconds for the small girl who was addressed as Uiharu to feel the breeze on her uncovered skin, to find that someone familiar just called out to her and flipped her skirt at the same time, and to finally widen her eyes when she saw that there were many people got a glance of her underwear.

The offender only smiled in amusement at the 'fabulous' sight before her. "I see that it's the classic blue-stripes for today, huh?"

"KYAAAH!" Uiharu Kazari screamed as she attempted to put her skirt back to place, making sure that her inner-parts were no longer visible from public eyes. She then turned her flushed face towards her apparent offender, who happened to be her close friend as well.

The said offender, as well as close friend was a girl with blue eyes and long black hair with a small, white flower on the left side of her hair. She was wearing the identical uniform like Kazari, and a wide grin was adorning her expression.

"Saten-san!" Kazari yelled as she flailed her arms agitatedly. "What are you doing to me?!" However, her outburst was too adorable to be intimidating in the eyes of most people.

"Sorry, sorry." The newcomer, Saten Ruiko apologized casually. "But come on, we're classmates, aren't we? No need to be so formal."

"Geez…. You're so mean." Kazari sighed.

"Like I said, I'm sorry." Ruiko replied. "You know what, how about I also show you mine as well?"

Kazari then blushed again. "No way, especially with so many people around!" the girl looked around to find that several boys of her school were either blushing, averting their gazes awkwardly or just outright grinned in rather perverted manner. "Anyway, let's just head to class!"

"Ah, wait up!" Seeing her friend quickly ran off to the school building, Ruiko also went after her.

* * *

"So, are you prepared?"

"Huh?"

"You know, the System Scan is coming soon, right? I ask if you're prepared for it."

Kazari then fidgeted slightly on her seat as she clarified Ruiko's question. "Well… I'm kind of nervous… But I guess I won't make much progress compare to last time…"

"Oh, come on…" Ruiko replied in comforting manner. "There has to be something. I mean, you're already a level 1! Even I believe that I'll definitely make some progress!" the girl finished with confident tone.

The city where the girls lived in had been conducting studies about psychic powers and abilities on their citizens, which 80% of the whole population happened to be students from all grades altogether. The psychics – Espers in the city were divided into different levels, from level 0 – the untalented and normal ones, until level 5 – the elite, powerful ones.

Uiharu Kazari was a level 1 esper, with her ability to be Thermal Hand – Fixed Temperature Preservation, which as the name implies, allows her to conserve part of the objects temperature she touches. Saten Ruiko on the other hand, was a level 0, while she possessed some decent athletic skills, she did not have any powers whatsoever.

The System Scan that the girl mentioned earlier was a regular city-wide testing event in Academy City to determine the power levels of the espers participating in it. Each esper would be given a different type of test depending on their power. The test then determines how powerful their ability is, how much control they have over it, and how easily it would grow.

Kazari could not help but smile at Ruiko's optimism. However, before the former could have said anything else in return, the latter had already beaten her to the punch already. "Speaking of upcoming events, have you heard that there will be transfer students coming here today? Two boys, to boot!"

Kazari then blinked at the news, which not exactly news to her. She recalled that according to the information she picked up from her line of 'work', there were two new transfer students coming from outside Academy City, from a town far away from Tokyo.

"Hm… I did read some information in their registry…" Kazari hummed. "I only know that those two transfer students are siblings; the older brother was supposed to be a second-year, but he has to repeat his first year again, the higher-ups saying something about the curriculum of his previous school is completely different than ours."

"Well, it's no biggie if there's only one year difference, right?" Ruiko replied. "Besides, there's a good chance that at least one of them might end up in our class! Man, this is so exciting! Mysterious transfer students, siblings to add! This feels kind of like a manga, don't you think?"

Kazari only laughed awkwardly at her friend's excitement. "Well… I don't think they are that mysterious to begin with…"

"Oh, oh! Did you get their names? Their faces?" Ruiko inched her face closer to Kazari's, which caused the latter to flinch back on reflex.

"Um… The older brother is Kageyama Shigeo, while the younger one is Kageyama Ritsu… As for their faces… They look normal, I guess?"

Ruiko pouted slightly at Kazari's answer before she pulled out her cell-phone and started to browse through internet.

"Saten-san, are you looking through that forum again?" Kazari asked, albeit she somehow could tell the answer.

"Well, it won't hurt to look from some other source, right?" Ruiko replied nonchalantly as she kept hitting her phone's buttons. "Hmm… The school they're coming from…. Oh, there's one!"

At Ruiko's finding, Kazari became curious as she inched closer to look at Ruiko's phone screen.

"Let's see… Wow, that's quite some rumour here…. These siblings are kind of famous from the town they came from." Ruiko summarised.

"Hm… Looks like that the younger brother was a member of the student council… That's pretty nice. And the older brother… it said that he's some kind of gang leader…?" Kazari could not help but frown slightly at the info. After all, she did not have a high opinion on delinquents.

"Shadow leader, apparently." Ruiko corrected. "And his nickname…. White T-Poison? What the hell's with that name?" the girl could not hold back her snort.

"The moniker came from the witness that claimed that the older Kageyama was wearing a white T-shirt when the fights occurred…" Kazari trailed off.

"Wow, check this out." Ruiko raised her eyebrows. "The site where the fight occurred had their buildings badly damaged… And they said that everything had been done by this White T-Poison guy alone…" the girl then turned to Kazari. "Uiharu, do you think that he's…?"

"Hm…" the level 1 tapped her finger on her chin in thinking manner. "It's not impossible, but it's still uncommon to have espers from outside Academy City."

"…Another esper, huh?" Ruiko guessed. "And according to the rumours, I guess it's safe to assume that the little brother is an esper as well. Whew, espers are just popping out everywhere." The girl said as she leaned back on her own seat.

"Well, those two will have to undergo the System Scan as well soon after they enrolled here, so we'll find out about them." Kazari said dismissively, before her expression went through another thought. "But it's really hard to believe that those two were involved in such things. They just don't look to be that bad…"

"You can't judge book from its cover, am I right?" Ruiko playfully commented.

It was a moment later that the door was slid open and a tall man with dark-bluish hair wearing glasses entered the classroom and took over the front desk. "Good morning, everyone. Take your seats!"

The students then obediently took their respective seats and proceeded with their formal greeting. The teacher took a quick look over his students, and he placed his hands on his desk to begin his morning announcement.

"Before we begin, there are few things I need to say." The teacher began. "First, I believe all of you are aware that the System Scan is coming soon. I hope that you all can make some progress in developing your powers, so please take this earnestly like you always did before. And secondly…. We have a transfer student in this class."

At the second news, students started to murmur and whisper to each other, obviously were curious and interested of their new potential friends.

"Alright, alright, calm down, all of you." The teacher clapped his hands to placate his students. "Our new friend is coming from outside Academy City, so he probably hasn't known his way around this place. So please go easy on him and be sure to show him around the ropes, okay?"

The class then let out affirmative replies, which the teacher returned with a pleasant smile and approving nod. Then, the teacher turned towards the door, which has a silhouette of a person on the glass.

"Okay, Kageyama-kun. You can come in, now."

As the teacher said it, the door then was slid open again, revealing a boy with dark bowl-shaped hair. As the new transfer student made his way to the front of the class, all students were staring at him. Everyone could not help but to think that the boy's appearance did not have anything impressive or standing out.

While Kazari was not the type to bother on someone's looks, Ruiko could not help but feel slightly disappointed at the boy's appearance _. "Wow… Uiharu was right… Forget normal, he looks completely like your average mob."_

"Well, Kageyama-kun. Please write your name on the board and introduce yourself, will you?"

"Uh, err… Yes." The boy was rather nervous as he began to write on the board with slightly trembling of his hand. Then he took a quick, deep breath and faced his new classmates with slightly pale and sweaty look on his expression. "M-my name is Kageyama Shigeo. This is my first time to be around this place, so please take care of me."

The mob finished his introduction with a bow.

 _Progress towards Mob's explosion: 7%_

* * *

 **Alright, as I have said before and on my profile, this is just a prototype and it's open for adoption. I may carry on with this story, or I may just drop it; because I want to focus on dealing with my One-Punch DxD project. But nevertheless, please review of what you think about how I've been faring so far.**

 **I actually considered to have Mob and Ritsu to transfer into different schools like Kamijou Touma, but it's not possible due to Touma being a high-school student. Of course, transferring them to Tokiwadai is out of question. So, I decided to have them to be in the same school with Uiharu Kazari and Saten Ruiko instead.**

 **As for the characters from Mob-Psycho 100, the reason I include Ritsu is simply because I want to play it safe. On that note, should I add more? And the next chapter (if I choose to continue this story) will explain a bit more about Kageyama's transferring into Academy City.**

 **On another note, there are few concerns:**

 **Should I cross Mob-Psycho 100 over with A Certain Scientific Railgun, which will focus more on science side, rather than To Aru Majutsu no Index, which will have Mob to fight with force of magic as well?**

 **What do you think of incorporating espers from Mob Psycho 100 into Academy City? Honestly, my knowledge on both franchises is very limited. Limited to anime and wikia for Mob Psycho case and limited to anime, manga and wikia in case of To Aru Majutsu no Index.**

 **What do you think about the levels for at least Kageyama siblings?**

 **Constructive opinions are appreciated, while flames are not acceptable. Ahsoei, over and out.**


	2. Chapter 1

**A Certain Unknown Mob**

* * *

 **A thousand apologies for the late update.**

 **First of all, thank you very much for reading and reviewing the prologue of this story. I'm very pleased at the positive review and encouragement to continue this story. Special thanks for max3213, The Another One, and several more guest and member reviewers alike that had given me some insightful opinion about Mob Psycho 100 and A Certain series.**

 **Secondly, as you can see, I have changed the title into something more suitable, 'A Certain Unknown Mob', since I myself still have no idea about Mob's true power/potential in ?% mode. Without further ado, on with the story!**

* * *

 **Beta read by Jack Rippling**

Chapter 1: The Arrival

In a certain windowless building at District 7 of Academy City, there was a room filled with several tubes and pipes covering the walls. It was the quadrangular room, and in the centre of that chamber, there was a large tube of red liquid with a figure of a human floating upside down and his body was connected to wires inside. The human figure looked like a tall effeminate-looking man with long white hair, wearing a green hospital gown. He looked like an adult, yet also a child; like a man, yet also a woman; like a saint, yet also a sinner at the same time.

The apparent man, Aleister Crowley, was smiling at the information he had received regarding a certain transfer student. The General Superintendent of Academy City had been aware of the existence of 'espers' outside his city. They were unique, strong in their own right, but at the same time, they were extremely naïve, pitiful, and most importantly, dull.

Aleister could not help but scoff at the memory of a certain man who happened to get over of his head, wanting to take over the world just because he possessed some significant powers over his peers. In fact, the man, Suzuki Touichirou once set his sight on Academy City. While his power was impressive, it only took several of Academy City's Rensa cyborgs to deal with him; vector manipulation and Dark Matter to overwhelm him, and some mind-tampering with Mental Out in order to have him to forget the existence of Academy City. Aleister Crowley did not have any intentions to involve such naïve and childish individuals for his plans.

However, among those espers, one caught his interest. The boy named Kageyama Shigeo. Not only he had so much potential, Aleister found that the boy had something resided within his body. Despite his powerful potential, Kageyama Shigeo stood out as he was entirely different from his peers. Perhaps if Aleister were to play his cards right, the young boy would be a valuable asset for his plans. After all, two spare plans were better than one. It might be fine to learn about the entity that resided within the boy's vessel.

* * *

It was during afternoon when a certain bus was moving on a seemingly empty highway. There were only two passengers in that bus; two boys, around 13-14 years of age, were sitting side by side. One boy possessed dark bowl-shaped hair, while the other possessed black hair that was slightly longer and more dishevelled. Both of the boys were wearing typical male gakuran. The two had been silent for a while, until one of them decided to break the ice.

"Doesn't this feel kind of exciting, nii-san (big brother)?" the boy with slightly longer hair spoke with soft tone.

"Eh? Um…" the other boys seemed taken aback by the sudden question, but he then nodded with a small smile on his face. "Yeah, I guess so."

The two fell into another silence, but it did not last long because the older of the two spoke this time. "This still feels kind of sudden, doesn't it, Ritsu?"

The boy, Kageyama Ritsu only hummed in return as he recalled the things that led him and his older brother, Kageyama Shigeo, into their current point.

Everything began when on one day, the two psychic brothers suddenly received a letter of offer and scholarship for them to be transferred and enrolled into a school within the Tokyo Metropolitan Employment Area.

His friends and family took this as a blessing and heavily encouraged the siblings to take the offer, but Ritsu was too smart and sceptical about the whole offer and scholarship thing.

Firstly, why him and especially his brother? Everybody knew that everything about Shigeo, save for his psychic powers, was normal. His academic grades were average, his physical capabilities were even worse, barely on average. Even Ritsu considered himself to be not exceptional enough to receive something like scholarship; everything was indeed sudden and out of nowhere.

Thus, Ritsu did a quick research about the destination where they were being transferred to. Upon the revelation, Ritsu could only think of a conclusion: their psychic powers were the main reason behind their transfer and scholarship. He had no definite proof, but there was no other logical answer, especially when he found out that the destination they were heading was conducting studies and researches about ESP.

Ritsu wanted to refuse the offer, but Shigeo, being an innocent and rather naïve boy he was, was excited and curious about the place that was said to have technology that was 30 years more advanced. Adding the constant encouragement from friends and family, the siblings decided to accept the offer. Besides, Ritsu thought it might be interesting to go to a new place where there were Espers everywhere.

Another thing that Ritsu noted was, that Hanazawa Teruki, Suzuki Shou, and several other Espers that he knew did not receive a similar offer, which was unexpected; especially considering that some of them were actually exceptional Espers in their own right. Was it because they managed to hide under the radar? Or whoever pulling the strings was not interested in them? He could only assume. Right now, all Ritsu could do was be ready for anything together with his brother.

"Yeah, it is." Ritsu replied, then he chuckled. "And what's more, they said that due to your grades were not adequate enough for you to enrol in the 2nd year, as well as the difference in curriculum, it seems that you have to start over from being a 1st year again."

Shigeo only sighed at his little brother's comment. He was shocked that he had to go back a year, but he knew better than to keep whining and lamenting about it. "But at least we're gonna spend more time together, Ritsu. Especially if we ended up being in the same class." The boy remarked with a smile, causing the younger one to smile contently at the optimism in return.

"Say, nii-san." Ritsu turned to his brother again. "What's the name of the new school we're enrolling again?"

Shigeo then proceeded to look into his bag, searching for the necessary paperwork. As he was rummaging through his belongings, Shigeo's mind was recalling the last conversations he had with his associates.

* * *

 _"Here's a friendly bit of advice from me, Kageyama…" Hanazawa Teruki said after he found out about the Kageyama's siblings sudden transfer. "I've been hearing that the place you and your little brother going to is the place where our powers are everything. If you keep stubbornly clinging to that principle of yours, you would probably get your brother and yourself hurt in the end."_

 _Shigeo only blinked and lowered his gaze slightly, processing what his fellow psychic had said to him. "Hanazawa-kun, did you also get an offer like us did?"_

 _Shigeo had no real idea about the reason behind his and his brother's sudden moving, but he just had a feeling that Teruki should have been involved as well._

 _"…Nah, it's not that I want to leave this place to begin with. I'm already rather attached to this place, even without using my powers for my own benefit." Teruki waved off. "Besides, the place where you're heading to just smells bad news to me."_

oXo

 _"I see, I think I'm gonna miss you a lot, Mob." Arataka Reigen said as he and his student were spending their time in their usual ramen shop for one last time in a while. "Business without you is gonna be tough…"_

 _"Master, will you be okay without me around?" Shigeo merely replied as he focused on his noodles._

 _"I'll be fine, I'll be fine…" Reigen sighed. "…I now have Serizawa, I think I can manage . Can't keep relying on my student, can I?" he finished with a small smile._

 _Soon after the meal, the master and apprentice were standing in front of the shop, ready to leave to their separate ways._

 _"Well, Mob. I don't know when and if we're gonna meet again, so until then, consider this my last words as your mentor."_

 _Shigeo then looked up, his attention was fully set on his 'mentor'. He had been thinking that Reigen might not be as he claimed to be, but even so, the fact remained that Arataka Reigen was a person by his own principle, and he had taught and guided Kageyama Shigeo to be the person he was now._

 _"I'm saying this because I've done a quick research about this Academy City." Reigen started. "Perhaps the things I'm gonna say to you will contradict with what I said before, but at least hear me out." Reigen then closed his eyes for few moments before opening them again._

 _"From what I gathered, psychic powers are everything in that city. There will be many people like you, maybe even stronger than you." Reigen began. "My point stands about having power not making you any more special than anyone else, but when you live there, you can't afford to pull your punches when the situation calls for it. But at the same time…" he then set his gaze directly at Shigeo. "...It will be up to you in the end, how you will adapt in Academy City. That's all I want to say."_

oXo

Shigeo just finished his trip down memory lane when he read the name of their new school on the paper in his hand. "It's… Sakugawa Middle School."

"I see…" Ritsu nodded. "I wonder if it's anything like our old school… Considering where we are now." he mused. Everywhere he looked, he saw many things that he could only see in science-fiction stories; wind turbines, several tube-looking robots rolling through the streets, cleaning up trashes off the road, people holding and playing at gadgets and cell-phones he had never seen before, even screens on buildings that he could only see at metropolitan Tokyo.

Shigeo only hummed in acknowledgement, and soon his lips grew into a small smile as he looked up ahead. "Let's do our best, Ritsu."

"Yes, I'm looking forward for this." the little brother nodded as his brother's attitude rubbed off him.

* * *

At Sakugawa Middle School…

"Kageyama-san, where did you come from?"

"Is it true that you received scholarship from board directors?"

"Were you actually a second-year?"

As soon as the home room ended, Shigeo was immediately surrounded by his new classmates who bombarded him with question after question. Being not the most social person, Shigeo was barely keeping himself together, as he would frantically look around the room every time a question was asked of him.

 _17%_

"Ah, err, um…" Shigeo became a stuttering mess as he was assaulted by one question after another. In the end, he could only look down with a pale face as his new classmates still surrounded him.

"Alright, alright, break it up, everyone!" a voice came to Shigeo's rescue as the students around him made a way for a new comer to face him. "The new guy is overwhelmed, see? Let's give him more space and ask one question at a time, okay?"

Shigeo's eyes were met by a girl wearing what appeared to be a set of flowers on top of her short hair. The said girl was accompanied by another girl with long hair and a single flower accessory standing by her side. When he looked at the face of the first girl, he found his cheek heating up.

 _"She's cute…. Even cuter than Tsubomi-chan…"_

 _20%_

"I'm sorry about them, we don't often have many transfer students from outside Academy City." The girl with headband smiled friendly at Shigeo. "My name is Uiharu Kazari, and this is my friend Saten Ruiko. Nice to meet you, Kageyama-san!"

"U-um, yes, nice to meet you too." Shigeo nodded back while his face remained flustered.

"So, Kageyama." Ruiko called his name casually. "What do you think of Academy City so far?"

That made Shigeo went silent for a few seconds in thought before replying. "It's…. completely different from my home in Spice City…. It feels like I'm in another world."

Shigeo's answer receiving several understanding and amused nods from his new classmates.

"Um…"

They then heard Shigeo calling out to them, his expression showed that he wanted to ask something. "I've been hearing rumours…." Shigeo went hesitant for a second before he decided to push through. "Is it true that there are espers here, in this city?"

Ruiko and Uiharu glanced at one another, and the two shared knowing smiles. It was normal for people from outside the Academy City to be marvelled by the main study field of Academy City - supernatural powers.

"There sure are; lots of them, Kageyama-san." Uiharu replied. "In fact, over 40% of the entire population in this city is consisted of Espers."

"Oh…" Shigeo's eyes widened and glazed over slightly at the information.

"What is this, Kageyama? Are you curious to see people with an actual super power, like in comic books?" One of the students asked with a knowing grin.

Shigeo blinked at his classmate, before he looked down to contemplate his answer. Should he just come clean? After all, he did reveal his power in front of his old friends back in Spice City without much difficulty. On the other hand, there was a reason why only a handful amount of people knew about his power.

"Well…." In the end, Shigeo decided his answer. "...It's because my brother and I are also Espers."

The room went silent for a few seconds at Shigeo's answer, before they erupted.

[Eeeh?!]

Shigeo jumped at the sheer volume of reaction around him. He looked around frantically, to see that his classmates were hounding him again with a newfound interest.

"Seriously?!"

"You have a psychic power too?"

"What's your ability?"

"An Esper outside of Academy City is not unheard of, but to see one in person… No, make it two with his brother in tow!"

"Wait, wait, hold up!" The class went silent at the shout, and they looked at one particular student who said the last line - a girl with a short, dark-brown hair. "Um, Kageyama-kun... Sorry for asking this, but can you prove it to us?"

When she received several looks from her own classmates, she quickly held her hands up defensively. "Look, no offence, but Esper from outside Academy City is rather uncommon, no? I just want a quick look of his claims; seeing is believing, right?"

Uiharu, Ruiko, and the other students glanced at one another, and a few seconds later they sent expectant looks at Shigeo. The boy simply blinked and stared at the girl who questioned him earlier, before he turned to look at his notebook on the table.

Suddenly, everyone's eyes widened when the notebook suddenly floated in the air. A minute later, Shigeo put the book back on the table, and he addressed his classmates. "Err… Is this enough?"

Murmurs began to erupt after they had witnessed Shigeo's psychokinesis.

"Whoa…"

"He's a real deal, alright."

"The way he control it… He's probably around at least level 2."

Shigeo only glanced around at his new classmates reviewing him. When he expected some degree of fear, Shigeo was surprised that every students' expression was that of genuine curiosity. Uiharu was impressed, while Ruiko had a strange look on her face.

Soon, however, the students began to surround Shigeo with a new batch of questions.

"Say, say, Kageyama. Since when did you have this power?"

"How heavy can you lift?"

"Do your parents and old friends know about this?"

"Are there more of people like you out there?"

Shigeo could only begin to panic and whimper again at the sheer amount of new students around him.

A long, first day at the new home awaited him.

* * *

Shigeo's rescue finally came in the form of his younger brother, who came to his classroom and took him out to have lunch together.

"It's been one helluva day, hasn't it, Nii-san?" Ritsu said as he took a bite of a sausage.

"Did they also ask you about your power too, Ritsu?" Shigeo replied with his own question, as he took a drink from his juice.

"Yeah, and I guess it really is normal to have psychic power here." Ritsu looked at the sky. "...It feels weird, though. We had to keep our powers secret back at home, but here…. We have to tell people. Well, more like report that we are Espers…"

The two brothers sighed at the same time, as they proceeded to eat in comfortable silence while watching clouds on the blue sky above.

 _So many things are going to change…_

* * *

 **I'm sorry that the update is short.**

 **The thing is, while I have no intention of abandoning this project, 95% of my muse had been utterly destroyed by the sheer of my disappointment of A Certain Magical Index Season 3 anime. Lazy animation, overly rushed plot, and no offence to Index fans out there, but she still annoys the crap out of me.**

 **The only silver lining of Season 3 anime is the fight between Accelerator vs Kakine Teitoku; which I still find not as good as expected. Although I gotta admit, the Awakened wings of both combatants looked pretty awesome.**

 **Constructive review will be highly appreciated. Ahsoei, over and out.**


End file.
